Fruit Trees Nutrition

Nitrogen Fertilization in Avocado - Tzabar Kama

Balanced nitrogen (N) fertilization is crucial for supporting the vigorous growth and reproductive development of avocado trees, but both N excess and deficiency can lead to reduced fertility. Recent studies on Hass avocado trees showed that while nitrogen did not significantly improve fertility, it did promote abundant flowering, which in turn led to aggressive leaf drop and drying of the inflorescences. Excess nitrogen has also been linked to internal-gray-pulp (IGP) and decay in fruits, particularly under low calcium conditions, which affect fruit quality and storage potential. Studies in Chile and South Africa have further linked excess nitrogen to IGP, and experiments in Israel found that both N deficiency and excess negatively impacted fruit size, color, cold tolerance, and postharvest quality.

The research suggests that nitrogen availability significantly impacts the fruit's chemical composition and levels of health-promoting compounds. This connection underscores the importance of optimizing nitrogen fertilization to enhance fruit quality and meet increasing market demand.

Wintertime Fertilization of Avocado

In recent years, there has been a rise in consumer demand for avocados due to their health benefits, including high levels of vitamins, antioxidants, and monounsaturated fats, which contribute to cardiovascular and anti-inflammatory benefits. As a result, global avocado plantations, especially of the Hass variety, have expanded significantly, though productivity in subtropical regions remains low. Research suggests that the fruit load in avocados is largely determined in late winter to spring when trees simultaneously flower, set fruit, and replace foliage. Many growers focus fertilization efforts during the warm months (April to October), but nitrogen (N) is often depleted in early spring due to leaching and nitrification. Experiments at the Gilat research center have demonstrated that high nitrogen levels (80 mg l-1) promote vigorous growth, abundant flowering, and high fruit set, while also encouraging the rapid growth of new leaves to protect developing fruitlets and support assimilate supply.

Response of Avocado to Mineral Nutrition Under Controlled Environment

Avocado’s high nutritional value and reputation as a health food have driven increasing demand both in Israel and worldwide. Avocado ranks among Israel's leading agricultural exports, with a recent surge in new plantations. Past research on avocado nutrition in Israel, focusing on freshwater irrigation, has shown promising results; however, most Israeli avocado orchards now use treated wastewater containing high levels of salts and nutrients. Reports and studies indicate that treated wastewater has altered soil characteristics, affecting tree development, causing chlorosis and salinity issues, and reducing fertility compared to freshwater. This shift in irrigation methods occurred without comprehensive studies on how to adapt nutrient management.

In response, the "Improving Avocado Fertility" project, led by Dr. Vered Yerachmilwitz, established two experimental systems in 2018 to investigate the Hass avocado’s response to various nutrients. Located at the Gilat Research Center and a commercial orchard near Kfar Menachem, these trials monitor water and mineral uptake, growth, and fertility under both controlled and field conditions. The combination of these studies aims to enhance nutrient management for treated wastewater irrigation, optimizing fertility and tree performance.

Avocado is now Israel's top agricultural export, generating over a billion shekels annually. Although the 'Hass' variety dominates exports and new plantings, it has relatively low fertility due to its sensitivity to extreme climates and pathogens. Beginning in 2019, controlled experiments with young Hass avocado trees have deepened understanding of their response to nitrogen, phosphorus, potassium, and micronutrient fertilization. Findings have raised important questions about the timing and impact of fertilization on fruit quality. The researchers plan to extend the study for three more years to better assess fertilization's effects on mature trees with significant fruit loads.

Vineyard’s Fertilization

Grapevine (Vitis vinifera) has spread globally, with vineyard areas and wine production steadily increasing. In Israel, wine grape vineyards span approximately 5,500 hectares from the Galilee to the Negev. These vineyards are often classified into quality or quantity-based categories, where high-quality vineyards yield lower quantities but have higher sugar and color levels, while high-yield vineyards produce more but with lower quality. These agronomic classifications guide vineyard management practices such as irrigation, pruning, and fertilization, although there are no specific fertilization guidelines based on these classifications.

Fertilization plays a significant role in grapevine growth, yield, and grape quality, impacting essential traits like sugars, organic acids, and flavonoids. Studies show that nitrogen affects amino acid levels and fermentation processes, while potassium supports sugar accumulation but can lower wine acidity if excessive. Phosphorus is linked to improved growth and can mitigate copper toxicity from fungicides. Current Israeli fertilization recommendations for nutrients vary, but they often overlook vineyard-specific factors like soil type, water quality, and regional characteristics.

The diversity of grape varieties, rootstocks, soil conditions, and agronomic classifications in Israel’s vineyards underscores the need for customized nutrient management approaches. Developing flexible, site-specific fertilization strategies that account for vineyard type, soil fertility, and product goals (e.g., red or white, high quality or high quantity) is crucial for optimizing vine health and grape quality.